The present invention relates to an operation of a non-volatile memory device, and more particularly, to a method of programming a flash memory device using self boosting.
A flash memory device, particularly a NAND flash memory device, has been increasingly used as a data storage medium. The flash memory device provides high memory density at a relatively low cost. Recently, in order to increase storage capacity on a small chip, a multi-bit cell capable of storing data of more than two bits in one memory cell has been developed. The memory cell of this type is generally called as a Multi-Level Cell (MLC). A Single Level Cell (SLC) includes one memory cell that has two states, i.e. program/erase. The MLC may store data of two bits, three bits, four bits or more with one memory cell. Thus, the MLC can realize a memory capacity of more than twice that of the SLC. The MLC generally has two or more threshold voltage distributions, and also has two or more data storage states corresponding to the threshold voltage distributions.
The NAND flash memory device is erased and programmed using Fowler-Nordheim Tunneling. During programming, a predetermined program voltage is applied to a word line of a selected memory cell and a ground voltage is applied to a bit line. In order to prevent programming an unselected memory cell, a power voltage is applied to the bit line. When the program voltage is applied to the word line of the selected memory cell and the ground voltage is applied to the bit line, a high electric field is formed between a floating gate and a channel of the memory cell. By this electric field, a tunneling, in which electrons of the channel pass through a tunnel oxide layer between the floating gate and the channel, is generated. By the accumulation of the electrons in the floating gate, the threshold voltage (Vt) of the memory cell is increased.
The distribution in the threshold voltage (Vt) of the memory cell in the programmed state in the NAND flash memory device is an important factor that influences performance of the device according to an over program problem and a read margin. The MLC flash memory device is programmed by an Incremental Step Pulse Program (ISPP) method, in which the program voltage is increased step by step according to a program loop. The ISSP method accurately controls the threshold voltage distribution of the memory cell. The program voltage (Vpgm) according to the ISPP method is increased step by step as program loops of a program cycle are repeated. Each program loop includes a program period and a program verify period. The program voltage (Vpgm) is increased by a determined step voltage (ΔV1), and the program time is maintained at a constant value with respect to each program loop.
In the MLC flash memory device, control for the threshold voltage of the programmed memory cell is a very important factor since the threshold voltage of the programmed memory cell is disposed so as to be spaced apart from each other between a first read voltage and a pass voltage (Vpass). However, it is very difficult to control a disturbance property due to interference between the memory cells. Also, a channel boosting is reduced as a coupling ratio is decreased according to a decrease in the cell size and, thus, it becomes more difficult to ensure the program disturbance property. The program disturbance is generated when the channel boosting is not enough. Particularly, a first word line is additionally subject to the disturbance as Hot Carrier Injection (HCI) by Gate Induced Drain Leakage (GIDL) is generated when channel boosting is too high.